The overall goal of this application is to identify the components and conditions for novel combination therapy of human myeloid leukemia. We will determine which combinations of silibinin, a plant-derived antioxidant, with vitamin D derivatives (VDDs) that have low cell toxicity and reduced calcemia-inducing activity, have potential as differentiation therapy for myeloid leukemia. Specifically, (i) we will optimize the conditions under which silibinin potentiates the induction of differentiation by VDDs in a range of myeloid leukemia cells. These cells will be from both established cell lines and from primary cultures of myeloid leukemia blood samples. Using these cells, we will also evaluate several promising newly synthesized analogs of vitamin D for their efficacy as differentiation-inducing agents in combination with silibinin. (ii) Secondly, we will utilize the information thus obtained to perform studies of the efficacy of these silibinin/ VDD combinations in a murine myeloid leukemia model, and in human myeloid leukemia cell xenografts. (iii) Thirdly, we will study the mechanisms of potentiation by silibinin of the action of the most effective VDDs. These experiments will address the hypothesis that the antioxidant action of silibinin amplifies the differentiation signal provided by 1,25-dihydroxyvitamin D and its low-calcemic analogs by generating a reducing intracellular environment, which then maintains transcription factors in their most active state in leukemic cells. These preclinical studies are designed to provide data that will permit the design of clinical trials of vitamin D analogs/antioxidant combinations in myeloid leukemia. Public Health Significance: Differentiation therapy, which depends on the activation of existing cellular programs rather than on toxic drugs to combat malignant tumors, is already effective as the treatment of some cancers. We propose to develop it as therapy for blood malignancy known as myeloid leukemia, which although kills approximately 15,000 people in USA every year, is unlikely to receive attention from commercial support for finding its cure. Thus, myeloid leukemia can be considered an orphan disease, and merits support from public sources for the development of novel therapy.